Using organic light emitting diodes (OLEDs) as monolithically integrated luminescent source in integrated optical devices might be beneficial in many applications for the following reasons: first, OLEDs can be fabricated by purely additive low-temperature processes such as ink-jet printing and can thus be easily integrated onto almost any substrate. Second, they are ultra-thin and thus offer the potential for being substantially space-saving. Third, via chemical modification of the OLED's light emitting molecule(s) the emission spectrum can be tuned over a wide range of the optical spectrum. Fourth, they are compatible with flexible polymeric substrates. Finally, due to their simple device architecture and processing scheme they hold the promise for low-cost fabrication and integration.
A major challenge pertaining to OLEDs is the coupling of the light emitted there into a low-order mode (less than 20 modes) supporting waveguide. Various publications teach how light emitted from an OLED may be coupled into a waveguide. However, implementations of these publications are not suitable for the coupling of light into a waveguide in low-order modes.
U.S. Pat. No. 5,907,160 discloses a thin film organic light emitting diode with edge emitter waveguide comprising, in sequence, a substrate, a waveguide, an anode, a hole transport layer, an electroluminescent layer, and a cathode. Voltage applied between the anode and cathode causes the electroluminescent layer to emit light through the hole transport layer and the anode into the waveguide where the light is internally reflected within the waveguide and propagates through the length of the waveguide to be emitted through the edge of the waveguide.
U.S. Pat. No. 6,472,817 discloses an organic light emitting device having a first electrode and a transparent electrode with an organic light emitting layer there between; characterized by a waveguide provided on the opposite side of the transparent electrode compared to the organic light emitting layer. In addition, U.S. Pat. No. 6,472,817 also discloses a device incorporating at least two such organic light emitting devices so as to provide a pulsed modulation output or a multi-color output.
U.S. Pat. No. 6,704,335 discloses an edge-emitting type light-emitting device that comprises an organic light-emitting layer, a pair of electrode layers for applying an electric field to the organic light-emitting layer, and an optical waveguide which transmits light emitted from the organic light-emitting layer to the edge. The optical waveguide disclosed in U.S. Pat. No. 6,704,335 further comprises a core layer mainly transmitting light, and cladding layers having a refractive index lower than that of the core layer. The core layer is a layer different from the organic light-emitting layer or comprises the organic light-emitting layer. A grating is formed in the core layer or in the boundary area between the core layer and the cladding layer. A light-emitting device comprises an optical fiber section. Another embodiment comprises a defect and a grating having a one-dimensional periodic refractive index distribution and constituting a photonic band gap. However, implementations of teachings disclosed in U.S. Pat. Nos. 5,907,160, 6,472,817 and 6,704,335 may induce waveguide losses caused by the presence of the OLED itself. Accordingly, implementations of the above-mentioned US patents fail to efficiently couple light emitted from the OLEDs into a low-order mode waveguide.
Further, Y. Ohmori et al. disclose in the publication “Realization of Polymeric Optical Integrated Devices Utilizing Organic Light-Emitting Diodes and Photodetectors Fabricated on a Polymeric Waveguide, IEEE Journal of Selected Topics in Quantum Electronics 10, p. 70, 2004” a 45° cut mirror at one end of a waveguide of 70 μm core size to reflect the light from the OLED on top of the waveguide into the core. However, since the optical power that can be coupled scales with the core size of the waveguide, the approach disclosed by Y. Ohmori et al. may yield insufficient power in the case of low-order mode waveguides.
Y.-Y. Lin et al. disclose in their publication “100 μm waveguide. 50 μm diffuser particles embedded in waveguide to enhance coupling efficiency, Applied Physics Letters 89, 063501, 2006” the introduction of a diffuser layer into the waveguide to couple light from an OLED into a coplanar waveguide. However, diffuser particles may be difficult to integrate into low-order mode waveguides of thicknesses equal to the wavelength of the light they guide. Furthermore, multiple scattering events may constitute a major problem in the case of low-order mode waveguides and limit the achievable coupling efficiency considerably.
U.S. Pat. No. 6,160,273A discloses an OLED-based edge emitter that mitigates the problem of unacceptably large losses by physically separating the OLED from the waveguide using an optical cladding layer. In their preferred embodiment the thickness of the cladding layer between the fluorescent medium/waveguide and the OLED is comparable to the wavelength of the photons generated in the fluorescent medium/waveguide. In their preferred embodiment the fluorescent medium also acts as a waveguide. In case the fluorescent medium cannot provide the required optical confinement, separate sub-structures of fluorescing material and light guiding material are proposed.